The present study was designed to evaluate the changes in the plasma lipid peroxidation and antioxidant system in 15 adult volunteer patients in hyperthyroid and euthyroid states. In these patients, plasma concentrations of lipid peroxides were decreased and, ascorbic acid and vitamin E levels were significantly increased in euthyroid status in comparison to hyperthyroid status. A significant increase in the plasma GPx activity (P < 0.01) and a decrease in GST (P < 0.001) was observed after euthyroidism was sustained with methimazole therapy. In conclusion, hyperthyroidism tends to enhance lipid peroxide content and an increase in GST and decreases in GPx, vitamin E and ascorbic acid levels accompany to this change in the plasma. The achievement of euthyroidism led an improvement in these parameters. (C) 1998 The Italian Pharmacological Society.